Lesson 4 — Expressions and Type Conversion

Estimated time: 60 minutes

Learning Objectives

By the end of this lesson you will be able to:

  1. Use arithmetic operators: +, -, *, /, //, %, **
  2. Understand operator precedence (order of operations)
  3. Convert between data types using int(), float(), str(), bool()
  4. Handle user input correctly (always comes as a string — convert it!)

Concepts

Arithmetic Operators

You already know +, -, *, /. Python has a few more:

Operator Name Example Result
+ Addition 7 + 3 10
- Subtraction 7 - 3 4
* Multiplication 7 * 3 21
/ Division 7 / 2 3.5 (always float!)
// Integer division 7 // 2 3 (drops decimal)
% Modulo (remainder) 7 % 2 1 (remainder after dividing)
** Exponent (power) 2 ** 8 256

/ always gives a float in Python 3 and MicroPython — even 10 / 2 gives 5.0, not 5. If you want an integer result, use //.

% (modulo) is particularly useful in programming. It gives the remainder after division:

  • 10 % 3 = 1 (10 ÷ 3 = 3 remainder 1)
  • 15 % 5 = 0 (15 ÷ 5 = 3 remainder 0 — divisible!)
  • Used to check if a number is even: if n % 2 == 0:

** (power) is useful for understanding colour depth: 2 ** 8 = 256, which is why each colour channel goes 0–255 (256 possible values).

Operator Precedence

Python follows BIDMAS/BODMAS — the same order of operations you learned in maths:

  1. Brackets first: (2 + 3) * 4 = 20
  2. Indices (powers): 2 ** 3 = 8
  3. Division and Multiplication (left to right)
  4. Addition and Subtraction (left to right)
print(2 + 3 * 4)    # 14 (multiply first, then add)
print((2 + 3) * 4)  # 20 (brackets first)
print(10 / 2 + 1)   # 6.0 (divide first, then add)
print(2 ** 3 + 1)   # 9 (power first: 8 + 1)

When in doubt, use brackets to make your intention clear.

Type Conversion

Sometimes you need to convert a value from one type to another. Python provides built-in conversion functions:

Function Converts to Example Result
int() Integer int("42") 42
int() Integer int(3.9) 3 (truncates!)
float() Float float("3.14") 3.14
float() Float float(5) 5.0
str() String str(42) "42"
str() String str(3.14) "3.14"
bool() Boolean bool(0) False
bool() Boolean bool(1) True
bool() Boolean bool("") False (empty string)
bool() Boolean bool("hi") True (non-empty)

int() truncates, it doesn’t round. int(3.9) gives 3, not 4. If you want rounding, use round(3.9) which gives 4.

Why Type Conversion Matters: input() Always Returns a String

This is crucial: input() always returns a string, even if the user types a number.

age = input("Enter your age: ")   # User types: 16
print(type(age))                   # <class 'str'> — not int!
print(age + 1)                     # ERROR: can't add str and int

You must convert it:

age = int(input("Enter your age: "))   # Convert immediately
print(age + 1)                          # Works: 17

Guided Walkthrough

Step 1: Try the Operators in the REPL

Click in the Thonny Shell and try each of these:

>>> print(10 + 3)    # 13
>>> print(10 - 3)    # 7
>>> print(10 * 3)    # 30
>>> print(10 / 3)    # 3.3333...
>>> print(10 // 3)   # 3
>>> print(10 % 3)    # 1
>>> print(2 ** 8)    # 256
>>> print(2 ** 10)   # 1024

Step 2: Operator Precedence 

# In the Script Editor:
print(2 + 3 * 4)       # 14 — multiplication first
print((2 + 3) * 4)     # 20 — brackets first
print(100 / 5 / 2)     # 10.0 — left to right
print(100 / (5 / 2))   # 40.0 — different!
print(3 ** 2 ** 2)     # 81 — powers go right to left: 3 ** (2**2) = 3**4 = 81
print((3 ** 2) ** 2)   # 81 — same here, but watch out in other cases

Step 3: Type Conversion 

# Strings to numbers
print(int("42"))        # 42
print(float("3.14"))    # 3.14
print(int("10") + 5)    # 15

# Numbers to strings
print(str(42))          # "42"
print(str(3.14))        # "3.14"
print("Score: " + str(100))   # "Score: 100"

# Truncation vs rounding
print(int(9.9))         # 9 (not 10!)
print(round(9.9))       # 10
print(int(-3.7))        # -3 (towards zero)

# bool conversions
print(bool(0))     # False
print(bool(1))     # True
print(bool(-5))    # True (any non-zero is True)
print(bool(""))    # False (empty string)
print(bool("hi"))  # True

Step 4: Converting input() — Essential Pattern 

# Always convert input() to the right type before doing maths
age_str = input("Enter your age: ")
age = int(age_str)   # Convert string to int
print(f"In 10 years you will be {age + 10}")
print(f"In 5 years you will be {age + 5}")
print(f"You were born around {2025 - age}")

Or do it in one line (more common):

age = int(input("Enter your age: "))

Step 5: LED Brightness Calculator

This program asks for a brightness percentage and converts it to the 0–255 range used by NeoPixels:

import machine, neopixel, time

pin = machine.Pin(48, machine.Pin.OUT)
np = neopixel.NeoPixel(pin, 1)

# Get percentage from user
percent = int(input("Enter LED brightness (0-100): "))

# Convert 0-100% to 0-255 range
# 100% = 255, 50% = 127, 0% = 0
brightness = int(255 * percent / 100)

print(f"Brightness: {percent}% = {brightness}/255")

# White at that brightness
np[0] = (brightness, brightness, brightness)
np.write()

time.sleep(3)
np[0] = (0, 0, 0)
np.write()

Notice: 255 * percent / 100 gives a float, so we use int() to convert it before using it as a colour value.

Step 6: A Colour Mixer 

import machine, neopixel, time

pin = machine.Pin(48, machine.Pin.OUT)
np = neopixel.NeoPixel(pin, 1)

print("Custom colour mixer!")
r = int(input("Red (0-255): "))
g = int(input("Green (0-255): "))
b = int(input("Blue (0-255): "))

# Clamp values to valid range (in case user enters out-of-range)
r = max(0, min(255, r))
g = max(0, min(255, g))
b = max(0, min(255, b))

np[0] = (r, g, b)
np.write()

total = r + g + b
average = total / 3
print(f"Colour: R={r}, G={g}, B={b}")
print(f"Total brightness sum: {total}")
print(f"Average brightness: {average:.1f}")   # :.1f = 1 decimal place

time.sleep(5)
np[0] = (0, 0, 0)
np.write()

Note the :.1f in the f-string — this formats the float to 1 decimal place. You’ll see this pattern a lot.

Challenges

⭐ Core

Write a program that asks for two numbers and prints their sum, difference, product, quotient (normal /), integer quotient (//), and remainder (%). Format the output neatly using f-strings.

⭐⭐ Extension

Ask for a number of seconds. Calculate and print: how many minutes and seconds that is (e.g., 150 seconds = 2 minutes 30 seconds). Then calculate hours, minutes, and seconds (e.g., 3665 seconds = 1 hours 1 minutes 5 seconds). Hint: use // and %.

⭐⭐⭐ Stretch

Ask the user to enter R, G, B values. Calculate:

  • The total brightness (R+G+B) and average
  • Whether the colour is “warm” (R > B), “cool” (B > R), or “neutral” (R == B)
  • The complementary colour (subtract each value from 255: comp_r = 255 - r) Display the original colour on the LED for 3 seconds, then the complementary colour for 3 seconds. Print both colours’ details.

Common Mistakes & Debugging

ValueError: invalid literal for int() with base 10: 'hello' You tried int("hello") — can’t convert non-numeric text to an int. Make sure the user entered a number.

ValueError: invalid literal for int() with base 10: '3.14' You tried int("3.14") — use float("3.14") first, then int() if needed, or use int(float("3.14")).

int() doesn’t round int(3.9) gives 3, not 4. Use round(3.9) to get 4.

Forgetting to convert input() age = input("Age: ") then age + 1 gives TypeError. Always convert: age = int(input("Age: ")).

ZeroDivisionError 10 / 0 or 10 // 0 — you can’t divide by zero! Check your divisor isn’t zero before dividing.

Key Vocabulary

Term Definition
expression A piece of code that produces a value (e.g., 2 + 3, len("hi"))
operator A symbol that performs an operation (+, -, *, /, //, %, **)
integer division (//) Division that drops the decimal part, always giving an int
modulo (%) The remainder after integer division
exponent (**) Raises a number to a power: 2 ** 8 = 256
operator precedence The order in which operators are applied (BIDMAS)
type conversion Changing a value from one data type to another
int() Converts a value to an integer (truncates floats — does not round)
float() Converts a value to a float
str() Converts a value to a string
truncate Remove the decimal part of a number (towards zero, not rounded)

Copyright © Paul Baumgarten.